1. Field
The present invention relates generally to wireless communication, and more specifically to an improved method and apparatus for space-time pre-correction of transmitted wireless signals.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards. Systems may incorporate support for delay-sensitive data, such as voice channels or data channels supported in the IS-2000 standard, along with support for packet data services such as those described in the IS-856 standard. One such system is described in a proposal submitted by LG Electronics, LSI Logic, Lucent Technologies, Nortel Networks, QUALCOMM Incorporated, and Samsung to the 3rd Generation Partnership Project 2 (3GPP2). The proposal is detailed in documents entitled “Updated Joint Physical Layer Proposal for 1xEV-DV”, submitted to 3GPP2 as document number C50-20010611-009, Jun. 11, 2001; “Results of L3NQS Simulation Study”, submitted to 3GPP2 as document number C50-20010820-011, Aug. 20, 2001; and “System Simulation Results for the L3NQS Framework Proposal for cdma2000 1x-EV-DV”, submitted to 3GPP2 as document number C50-20010820-012, Aug. 20, 2001. These are hereinafter referred to as 1xEV-DV. Non-CDMA systems include the AMPS and GSM systems.
Multipath is a condition that occurs when a transmitter transmits a single signal that is then received at a receiver through multiple signal paths, each having a different length. The difference in the lengths of the paths may cause different copies of the signal to arrive at different times, causing inter-channel interference. Various techniques are known in the art for combating multipath interference. One example is a RAKE receiver. A RAKE receiver attempts to separate interfering multipaths and combine them to improve receiver performance.
Given the limited amount of spectrum available to communication carriers, it is desirable to increase the data throughput for the given amount of power in a given frequency band. Doing so may increase data rates, increase capacity, and/or reduce power (and potentially reduce costs in communication equipment). For example, due to expected asymmetric data rates on the forward link versus the reverse link of a CDMA system, there is interest in increasing the forward link system capacity and/or data throughput via spatial signal processing techniques. For example, transmit antenna arrays are being deployed to achieve gains.
However, it would be desirable if multi-path interference could be reduced or eliminated before it occurred, thus reducing or eliminating the need for multipath mitigating processing at the receiver, as well as improving the overall capacity and/or throughput of the system. There is, therefore, a need in the art for pre-correction of transmit signals to reduce multipath interference.